In supercharged internal combustion engines, compressed air is provided to the cylinders via an appropriate air feed system. The air is compressed with the aid of supercharging devices such as turbochargers, for example. When the air is compressed, it is heated, which without further measures would result in reduced efficiency of the combustion of an air-fuel mixture which is produced using this heated compressed air. To increase the efficiency of the combustion, the charge air which is heated by the compression of the ambient air may be cooled with the aid of a so-called intercooler.
In particular for an internal combustion engine which is cold, i.e., in its start-up phase, cooling of the charge air is not advantageous because it delays the heating of the internal combustion engine to its operating temperature. However, rapid heating of the internal combustion engine to operating temperature is desirable for reducing harmful emissions. For this reason intercoolers are generally provided with a bypass line in which a bypass valve is situated. The bypass valve is actuated in such a way that when the internal combustion engine is cold (i.e., a short time after the internal combustion engine is started in the cold state), the valve is opened to divert the compressed intake air around the intercooler, thus avoiding cooling. As a result, the internal combustion engine is heated more rapidly. If the internal combustion engine is at operating temperature, the bypass valve is closed, and therefore the intake air is completely led through the intercooler. Depending on the operating state, intermediate positions of the bypass valve may also be provided, depending on the design of the engine system.
To detect impairment of the operation of the internal combustion engine, which increases harmful emissions, as the result of a stuck bypass valve, an onboard diagnosis system may be implemented which actuates the bypass valve when certain enabling conditions are present, so that the valve closes and reopens for a specified period of time, during which the temperature behind the intercooler, i.e., downstream from the intercooler, is monitored. For both time periods a temperature gradient is generated, and is checked for plausibility in order to detect proper or improper functioning of the bypass valve.
For enabling the onboard diagnosis, previous diagnoses must show that the sensors and actuators used are not defective, and that a specified operating point is maintained. The operating point may be a function of the engine rotational speed, injection quantity, ambient temperature, instantaneous air mass, charge pressure, vehicle speed, and the engine operating mode. Plausibility checking is carried out by generating an average temperature gradient based on the difference between the initial and final temperatures downstream from the intercooler during the particular time period, and comparing the absolute value to a threshold value. If the gradient is too small, the bypass valve is recognized as defective.
Depending on the operating state, when diagnosis is enabled in the steady state, under some circumstances a temperature gradient downstream from the intercooler when the bypass valve is open or closed cannot build up, so that a bypass valve stuck in a position cannot be detected in this manner.
If the diagnostic conditions are set in such a way that the enabling takes place in the dynamic case (for example, for a high charge pressure and large injection quantities), a temperature gradient may be determined only when the enabling conditions last for a sufficient length of time. However, since in the dynamic case the charge pressure and the injection quantity may change rapidly, the diagnosis may possibly be terminated before the entire sequence of opening and closing of the bypass valve and computing the resulting temperature gradients is completed. Therefore, it cannot be ensured that the diagnosis of the bypass valve for the intercooler is carried out within a normal driving cycle, since it is possible that enabling conditions may not be met for a sufficient contiguous period of time during the entire operating period.